Phase-shifting transparent lithographic mask for writing contiguous structures from noncontiguous mask areas

Abstract: A plurality of noncontiguous polygonal regions of phase-shifting material have edges spaced apart less than the distance at which images of said edges would separate. The edges of the regions of phase-shifting material have differing arbitrarily selectable angular orientations (nonparallel as well as parallel) on a nonphase-shifting material. The noncontiguous phase-shifting regions may be arranged in a matrix of parallel columns and rows to facilitate fabrication and facilitate writing of the phase-shifting regions in any arbitrary pattern on an image plane. At least one of the phase-shifting regions constitutes a connective phase-shifting region with edges spaced from edges of adjacent ones of the phase-shifting regions to create a pattern with a continuous body of photoresist covering areas corresponding to the phase-shifting regions upon exposure of the mask. The phase-shifting regions may have different transmission coefficients to provide differing degrees of transparency or different degrees of phase shift to provide differing degrees of image intensity.

Description: This invention relates to phase-shifting lithographic masks with transparent portions providing regions of nonphase-shifting material covered in part by regions of phase-shifting material. Phase-shifting masks have recently been found to facilitate fabrication of new generations of integrated circuits and dynamic random access memories (DRAMs). In phase-shifting masks for exposing a photoresist pattern, phase-shift material delays the electromagnetic wave passing through the pattern so it arrives substantially 180° out of phase with the wave passing through the transparent apertured areas of a nonphase-shifting material. (As herein used, the term "nonphase-shifting material" connotes a material that introduces a constant phase shift, and the term "phase-shifting material" connotes one that introduces an additional phase shift due to a difference in thickness or refractive index gradient or a combination of both.) This creates destructive interference between the images of the nonphase-shifting regions and the phase-shifting regions which enhances the contrast of the overall image. It is known that the dark fringe from two closely-spaced parallel dark phase-shifted edges can be darker than the image of a single narrow opaque line and thus provides what has been termed a "darker-than-dark" mask. The exact conditions under which the two edges merge depend upon the resist processes used for lithography, but generally occur for mask features with k 1 between 0.35 (when the fringe is barely dark enough to print) and 0.6 (when the two edges separate into two distinct dark lines). The width w of a feature as optically imaged is given by w = k 1 λ NA where NA represents the numerical aperture (typically 0.45 for modern i-line steppers) and λ is the wavelength (λ = 0.365µm for i-line steppers). Phase-shifting masks may or may not include a chrome layer. EP 0 464 492 A1 discloses a photomask used by photolithography and a process for producing the same which allows a single exposure to make a mask, thereby simplifying the mask making process, and facilitating the inspection and correction of photomasks. In addition the phase shifter using a slanting pattern prevents a pattern from being formed outside a predetermined area. The use of a phase shifter which does not resolve under an optical projection system shield a large size area against an irradiated light, thereby allowing the formation of fine, intricate patterns suitable for use in LSIs. Toh et al., "Chromeless Phase-shifted Masks: A New Approach td Phase- shifting Masks", presented at the Tenth Annual Symposium on Microlithography of BACUS, September 1990 discloses transparent (i.e., chromeless) phase-shifting lithographic masks that write patterns of contiguous structures from closely-spaced regions of phase-shifting material covering selected areas of a transparent nonphase-shifting material. One embodiment (at the top of Fig. 13) is a mask with two integral orthogonally arranged phase-shifting regions that form a right-angle elbow; but (as shown at the bottom of Fig. 13) the image intensity is nonuniform because the diagonal distance between the inner and outer edges at the junction of the elbow is greater than that between the parallel edges of the straight sections of the elbow. If the diagonal distance is great enough, the images of the phase-shifting regions will not merge at the junction and the bright spot thus created will cause an undesired removal of photoresist at the junction. Another embodiment (shown in Figs. 14 and 15) depicts a mask layout pattern consisting of a "checkerboard" arrangement of phase-shifting and nonphase-shifting regions in which the phase-shifting regions have contiguous corner contact with those in adjacent rows and columns. Watanabe et al., "Sub-quarter-micron Gate Pattern Fabrication Using a Transparent Phase Shifting Mask", published by the Journal of Vacuum Science Technology, Vol. B9, No. 6, November/December 1991, pp. 3172-3175 discloses (in Fig. 5) a plurality of spaced parallel regions of phase-shifting material. According to the present invention there is provided a mask for exposing photoresist, the mask comprising a nonphase shifting region and covered in part by a plurality of non-contiguous phase shifting regions each of the phase shifting regions having edges spaced from each other by distances less than that at which images of said edges separate. The mask is characterised in that said plurality of non-contiguous phase shifting regions are in the form of substantially equally-spaced circles arranged in columns and rows to facilitate fabrication and to facilitate writing of said phase-shifting regions in any arbitrary pattern on an image plane, each of said hollow circles having circumferential edges spaced apart from circumferential edges of respective adjacent circles by distances less than that at which images of said edges separate. The invention also provides a method for creating a mask for exposing photoresist, comprising the steps of writing a pattern onto a chrome-quartz substrate, etching the chrome-patterned substrate in an etching solution, and removing the chrome from the substrate to produce a phase shifting pattern on said substrate, the phase shifting pattern being formed by a nonphase shifting region covered in part by a plurality of non-contiguous phase shifting regions each of the phase shifting regions having edges spaced from each other by distances less than that at which images of said edges separate. The method is characterised in that said pattern produced by removing the chrome from the substrate produces a plurality of non-contiguous phase shifting regions, in the form of substantially equally-spaced circles arranged in columns and rows to facilitate fabrication and to facilitate writing of said phase-shifting regions in an arbitrary pattern on an image plane, each of said hollow circles having circumferential edges spaced apart from circumferential edges of respective adjacent circles by distances less than that at which images of said edges separate. The noncontiguous phase-shifting regions are arranged in columns and rows to facilitate fabrication and facilitate writing of the phase-shifting regions in any arbitrary pattern on an image plane. The invention provides transparent lithographic masks and methods of fabricating same which (1) can provide images with wider line widths than the prior art; and which, in addition, (2) can provide improved depth of focus and/or more latitude in times and intensities in the exposure processing window; (3) enable the mask for certain applications to be written with only one exposure; and (4) enable optical inspection techniques without requiring subresolution to check integrity of the masks in a single electron beam write operation. Embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which: Fig. 1A depicts a prior art transparent mask with two closely-spaced sets of parallel edges for providing a dark "L" or elbow-configured image; and Fig. lB depicts the image, including a bright spot, which may result when the mask is exposed. Fig. 2A depicts a transparent mask with a grid pattern of phase-shifting regions, according to an embodiment of the invention; and Fig. 2B depicts the image provided by the mask of Fig. 2A when exposed. Figs. 3A and 4A depict other transparent masks generated from grid patterns of phase-shifting regions; and Figs. 3B and 4B depict the images provided by the masks of Figs. 3A and 4A, respectively. Each of the masks 10 hereinafter described embodying the invention consists only of phase-shifting regions 12 etched in selected patterns in a transparent nonphase-shifting material 14, such as quartz. Fig. 1A depicts a prior art mask 10a having an "L"-shaped mask pattern comprising two pairs of parallel lines that intersect at right angles. This pattern is intended to provide on a substrate 15a a completely dark L-shaped image 16a when exposed, but, as illustrated in Fig. 1B, is flawed with a bright spot 18. This bright spot can result from the fact that while the distance d between the parallel edges (i.e., the width) of each leg 20, 22 of phase-shifting material is small enough to ensure that the image in both legs of the L will be written completely dark without blemish, the diagonal distance at the junction of the orthogonally arranged, interconnected legs 20, 22 is much as d√2 . This may be enough to exceed a minimum distance d min at which edge separation takes place, noting that d min will vary depending upon the lithography exposure tool used and the exposure characteristics of the photoresist. The obvious solution would be to reduce the distance d so that the distance d√2 would be within the aforesaid minimum distance d min. However, this will undesirably reduce the maximum printable line width achievable. As illustrated in Fig. 2A, according to an embodiment of the invention, mask 10e has a plurality of phase-shifting regions 12e arranged in a grid-like or matrix arrangement of columns and rows. Regions 12e have circumferential edges spaced apart from circumferential edges of respective adjacent regions 12e by distances less than the minimum distance d min. As a result, and as depicted in Fig. 2B, the mask pattern will print on substrate 15e a single, substantially rectangular, completely dark image 16e. In practice, mask 10e was fabricated from a layout of hollow 0.75 µm squares arranged in a matrix pattern and having a pitch of 1.75 µm; i.e., with a space of 0.25 µm between the edges of adjacent squares. These squares etched into the circular phase-shifting regions 12e are approximately 1.2 µm in diameter. This mask was subsequently printed on a Nikon NSR tool with a five-times reduction. The pattern on mask 10e printed as opaque for exposures up to eight times the normal exposure dose needed to clear photoresist on silicon wafers, indicating that the mask pattern was highly efficient. Micrographs of the exposed silicon wafers demonstrated this insensitivity to overexposure. The mask printed as opaque for several µm of defocus as well. According to a feature of the invention, mask 10e with its matrix arrangement of noncontiguous phase-shifting regions 12e is easier to fabricate than one having a checkerboard arrangement of contiguous phase-shifting regions. Also, the mask 10e may be patterned in any desired configuration, in much the same manner as a monitor screen consisting of a dense matrix of picture elements or "pels". By using noncontiguous phase-shifting regions etched in selectable rows and columns, it is possible to readily generate mask patterns with any desired configuration. For example, the pattern on mask 10f of noncontiguous phase-shifting regions 12f illustrated in Fig. 3A will produce on a substrate 15f the image 16f shown in Fig. 3B. The pattern on mask 10g of noncontiguous phase regions 12g illustrated in Fig. 4A will produce on substrate 15g the image 16g shown in Fig. 4B. In each case, the respective regions 12f and 12g will be separated from each other by distances not exceeding the minimum distance d min for the lithography exposure tool used and exposure characteristics of the photoresist. An important consideration in the design of chromeless phase-shifting masks is to utilize fabrication processes which can be easily reproduced. A key step in such a process is etching the mask substrate material to generate topography which results in the desired phase-shifted regions. While dry etch processes (reactive ion etching, or RIE) seem initially attractive, such processes are difficult to control to provide uniform etch rates (<5% etch depth variation) over the large areas required to fabricate a 12.7x12.7 cm (5x5-inch)mask substrate. RIE processes also tend to produce roughness in topography in the bottoms of the etched areas. Applicants have found that a wet etch process has the advantage of providing a smooth (at the 0.5-1nm (5-10Å)level) topography in the etched areas. More specifically, the following steps were followed to fabricate applicants' transparent phase-shifting masks. A desired pattern was written on a chrome-quartz substrate by electron-beam lithography or laser lithography. Then the clean chrome-patterned mask was immersed in a solution of a commercial 5:1 buffered hydrofluoric acid (mole ratio of saturated aqueous ammonium bifluoride/hydrofluoric acid) "silicon oxide etchant" maintained at a temperature of 17-20°C. Since the etch rate is strongly dependent on temperature, the etch time had to be adjusted for each experiment if the etchant temperature was not controlled to ±0.2°C for each etch run. Once etched for the appropriate length of time (by empirically correlating the etch rate for the feature size being etched with the desired etch depth), the mask substrate was removed from the etch solution and immediately rinsed with deionized water for at least five minutes to remove all traces of etchant. In practice, an etch time of 170 seconds was required to achieve an etch depth of 346nm (3460Å). The chrome was then stripped from the mask to provide the desired phase-shifting pattern on the quartz substrate. With edge depths of 250-400 nm (2500-4000Å) and a feature size ≥ 0.4µm, this etch process provided masks having significant anisotropic characteristics. The etched walls were surprisingly and desirably found to produce rounded corners and to be somewhat more vertical than would be expected with a totally isotropic etch process. Applicants' transparent phase-shifting masks may thus be written with only one exposure. However, if phase-shifting regions with different transmission coefficients are desired to provide differing degrees of transparency, a separate exposure step will be required for each different transmission coefficient. Variations in transparency are achieved by providing a substrate comprising quartz and chrome or another opaque material having differing chrome thicknesses. Also, by providing phase-shifting regions with different degrees of phase shift, differing image intensities can be achieved. While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.

Claims: A mask for exposing photoresist, the mask comprising a nonphase shifting region (10e,10f,10g) covered in part by a plurality of non-contiguous phase shifting regions (12e,12f, 12g) each of the phase shifting regions having edges spaced from each other by distances less than that at which images of said edges separate; characterised in that: said plurality of non-contiguous phase shifting regions (12e,12f,12g) are in the form of substantially equally-spaced circles arranged in columns and rows to facilitate fabrication and to facilitate writing of said phase-shifting regions in any arbitrary pattern on an image plane, each of said hollow circles having circumferential edges spaced apart from circumferential edges of respective adjacent circles by distances less than that at which images of said edges separate. A mask as claimed in claim 1, wherein at least two phase shifting regions (12e,12f,12g) have different transmission coefficients to provide differing degrees of transparency. A mask as claimed in any preceding claim, wherein at least two of the phase shifting regions (12e,12f,12g) have different degrees of phase shift to provide differing image intensities. A mask as claimed in any preceding claim, wherein at least one phase shifting region (12e,12f,12g) provides a phase shift of substantially 180 degrees relative to the non-phase shifting region. A method for creating a mask for exposing photoresist, comprising the steps of writing a pattern onto a chrome-quartz substrate, etching the chrome-patterned substrate in an etching solution, and removing the chrome from the substrate to produce a phase shifting pattern on said substrate, the phase shifting pattern being formed by a nonphase shifting region (10e,10f,10g) covered in part by a plurality of non-contiguous phase shifting regions (12e, 12f, 12g) each of the phase shifting regions having edges spaced from each other by distances less than that at which images of said edges separate; characterised in that: said pattern produced by removing the chrome from the substrate produces a plurality of non-contiguous phase shifting regions (12e,12f,12g), in the form of substantially equally-spaced circles arranged in columns and rows to facilitate fabrication and to facilitate writing of said phase-shifting regions in any arbitrary pattern on an image plane, each of said hollow circles having circumferential edges spaced apart from circumferential edges of respective adjacent circles by distances less than that at which images of said edges separate. A method as claimed in claim 5, wherein the etching solution comprises hydrofluoric acid and a buffer comprising a saturated aqueous ammonium bifluoride, and including the step of selecting the ratio of hydrofluoric acid to the buffer and selecting exposure time and temperature to provide a desired phase shift and wall angle.

Inventors: BROCK PHILLIP JOE, FRANKLIN JACQLYNN ANN, SCHELLENBERG FRANKLIN MARK, TSAY JIUNN, BROCK, PHILLIP JOE, FRANKLIN, JACQLYNN ANN, SCHELLENBERG, FRANKLIN MARK, TSAY, JIUNN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1994
Main classification: G03F1
IPC codes: G03F7, G03F1